1. Field of the Invention
This invention relates to a new type of meter which, by using ultrasound and adopting suitable expedients associated with a particular geometry of the measurement tube, ensures under all conditions a constantly laminar flow with a parabolic fluid velocity distribution within said tube to hence enable the emitter-sensor pairs for the ultrasonic beam to be positioned aligned along the tube axis where the signals are least disturbed and of greatest intensity, and in addition allows not only an economical and highly precise measurement of the flow rate of any fluid to be achieved but also allows said flow rate to be easily and precisely controlled and the flow to be instantaneously shut off, as will without doubt for safety reasons be required of a later generation of gas meters for domestic and in particular industrial use.
2. Discussion of the Background
In the present state of the art, for such gas meters mechanical systems are used which operate by displacement of gas volumes within one or more chambers of a bellows type and hence are of variable geometry, by means of a rotary distributor or reciprocating slide valves, the flow rate being proportional to the volumetric displacement of said chambers.
Such known meters have however the drawbacks of a mechanically complex and hence costly structure, plus moving parts such that the inevitable wear and soiling thereof can prejudice measurement accuracy. Again, their exquisitely mechanical nature makes them particularly unsuitable for development as an "intelligent" meter without further costly constructional complications. From the state of the art there are also known various types of systems for measuring the flow of gas and fluids in general based on the ultrasound principle, ie determining the variations in the time taken by ultrasonic beams to pass from emitters to sensors or receivers variously positioned relative to the axis of the measurement tube.
Although said ultrasound principle is intrinsically insensitive to dirt and suitable for "intelligent" development of the relative measurement system, it has however the serious drawback of being strongly influenced by the type of motion of the fluid within the tube, and this in particular along the tube axis, so that each time there is a discontinuity in the fluid motion in the sense of passing from laminar to turbulent flow, the flow measurement is affected by an error emitting from the change in the fluid velocity distribution from a parabolic to a flat profile. Again, the presence of a flow which can be randomly laminar or turbulent always creates considerable problems of measurement precision and instability, in addition to the fact that by generating vortices, turbulence introduces without doubt a further disturbance in the ultrasound movement.
To obviate said turbulence drawbacks said known ultrasound measurement systems use various expedients such as not orientating the ultrasonic beam in the direction of the tube axis, this being the region in which greatest velocity change occurs in passing from the laminar to the turbulent regime, even though it is precisely in this region in the case of laminar flow that the measurement can be made under the best possible exactness and precision conditions in that the emitted signal reaches the sensor practically undeformed or with minimum distortion, this orientation instead being offset from said axis by slightly more than one half the tube radius, to operate in regions in which the flow velocity is approximately equal for both laminar and turbulent flow (see in this respect U.S. Pat. No. 4,078,428 of the National Research Development Co. and U.S. Pat. No. 4,102,186 of E. I. Du Pont de Nemours & Co.).
However, with these offset measurements the threads of the ultrasonic beam travel at different speeds to reach the sensor at different times, to hence distort the transmitted signals and require the use of complicated and costly handling of said signals together with a multiplicity of measurements to achieve an average approximating to the correct flow value.
Hence, all known ultrasonic fluid flow measurement systems have limits due to high cost because of the need for a multiplicity of emitters and sensors, the burden and high energy consumption involved in the complicated processing of data to achieve a measurement substantially independent of the law of fluid motion, and the inconvenience of an approximate measurement obtained as the average of several measurements.